Low density lipoprotein in blood. Illustration of low density lipoprotein (LDL), or 'bad' cholesterol, in the bloodstream. This biochemical assembly consists of a core of esterified cholesterol molecules surrounded by a shell of unesterified cholesterol (orange with violet cap) and phospholipids (orange with blue cap). The complex structure includes carrier proteins (blue) known as apo-B proteins, which assist transport in the blood. When there are high levels of LDL cholesterol in the blood it builds up on the sides of blood vessels hardening them, a condition named atherosclerosis. This narrows the blood vessels and may block them.